The phrase "mat up" can have a variety of synonyms depending on the context and intended meaning. In terms of physical objects, synonyms might include "knit together," "tangle," "interweave," or "felt." When referring to hair, "mat up" might be replaced by "tangle," "knot," "clump," or "snarl." In a figurative sense, synonyms for "mat up" could include "complicate," "entangle," "conflate," or "confuse." Ultimately, the best synonym for "mat up" will depend on the specific usage and intended meaning, but keeping these options in mind can help you convey your ideas more accurately and effectively.